1. Field of Invention
The present invention relates generally to the field of cardiac defibrillation and in particular to a method and apparatus for applying to the body of a patient a voltage calculated to induce in that particular patient a desired defibrillation current.
2. Description of the Prior Art
A defibrillator is a device used to administer a high voltage shock pulse through a pair of electrodes, or "paddles," to the chest of a patient in cardiac arrest. A selected, discrete quantity of energy is stored in a capacitor and is then electrically discharged into the patient through the paddle circuit. In prior art devices, the quantity of energy is typically selected on the basis of patient size, weight and condition.
The success achieved with such prior art devices has been variable. While an energy of 150 joules might successfully defibrillate one patient, another ostensibly comparable patient may not respond until the energy level is raised to 360 joules. The risk of damage to the myocardium from large defibrillation shocks dictates that defibrillation should be attempted with the lowest energy practicable. However, delays associated with charging the defibrillator to successively higher energy levels if lower energy pulses fail increase the risk of patient brain damage due to oxygen deprivation.
The differing responses of patients to a given defibrillation pulse is believed attributable, in significant part, to patients' differing transthoracic impedances. Transthoracic impedance is the term given the load resistance presented by a patient at the paddles during defibrillation and determines, inter alia, the fraction of energy stored in the storage capacitor that is actually delivered to the patient. One type of known defibrillator, shown in U.S. Pat. No. 4,328,808, is able to compute the transthoracic impedance of the patient being defibrillated from measured pre- and post-discharge parameters. This information can be used by the operator as an aid in selecting the energy of a second defibrillation pulse if the first pulse proves ineffective.
In an article by Kerber et al. entitled "Automated Impedance-Based Energy Adjustment for Defibrillation: Experimental Studies," Circulation, Vol. 71, No. 1, pp. 136-140 (1985), there is described an experimental animal defibrillator in which transthoracic impedance is detected in advance of defibrillation by using a low current A.C. sampling signal. The defibrillation energy level selected by the operator is automatically increased, generally by a factor of two, if the detected patient impedance exceeds 70 ohms. Although an improvement over prior systems, the Kerber et al. system still suffers from a number of deficiencies making it poorly suited for clinical application.
Principal among the deficiencies of the Kerber et al. system is its lack of control over current delivered to the patient. Our work has indicated that defibrillation success as a function of current is optimized in humans about a narrow range of current values for a given pulse shape. For a pulse duration of four milliseconds, the optimum current is approximately 30 amperes. Currents only slightly below this value have a much poorer success rate. The magnitude of transthoracic impedance encountered in typical patients, however, ranges from approximately 15 to 150 ohms, a 10 to 1 ratio, rendering the delivery of an optimum current unlikely if the defibrillator can only alter the operator-selected energy to a single other value.
Accordingly, a need exists for a defibrillation method and apparatus that permits selection of a determined defibrillation current for a patient in cardiac arrest, irrespective of the magnitude of that patient's transthoracic impedance.